1. Field of the Invention
The present invention relates to an improved oxidation procedure in the Witten-Hercules process for preparing dimethyl terephthalate (DMT).
2. Disccusion of the Background
In the Witten-Hercules process for preparing DMT, p-xylene (pX) is oxidized in one or more stages with oxygen-containing gases, such as air, in the presence of cobalt catalysts and manganese catalysts to give p-toluic acid (pTA). The pTA is then esterified with methanol to give methyl p-toluate (pTE), which is recycled to the oxidation, where it is oxidized to give terephthalic acid monomethylester (monomethyl terephthlate; MMT). The oxidation product pTA still has one further oxidizable methyl group and is therefore oxidized to terephthalic acid (TPA) like pTE to MMT under very similar conditions. However, since the melting temperature of pTA is higher than the oxidation temperature and the resulting TPA cannot be melted and is poorly soluble in the reaction medium, this reaction may only take place to a restricted extent, since the oxidized product can otherwise no longer be handled. It is therefore characteristic of all embodiments of the Witten-Hercules process that pTE is oxidized in a competing reaction to MMT. MMT is considerably more soluble than TPA, and pTE is a good solvent for pTA and MMT, has a low melting point and, even in relatively low amounts, considerably depresses the melting temperature of the oxidized product.
A comparable action is exerted by recirculated products which originate from subsequent stages of the DMT process. In addition to the highly predominant contents of substances neutral to oxidation, such as methyl benzoate (BME), DMT, dimethyl o-phthalate (DMO) and dimethyl isophthalate (DMI), the recirculated products have minor amounts of oxidizable contents, such as methyl terephthalaldehydrate (TAE) and methyl hydroxymethylbenzoate (HMBME). Particularly active compounds in this context are DME, DMT and DMI. If these recirculated products are introduced into the oxidation of pX, the amount of pTE also introduced can be kept lower. This is desirable, because separating off excess, (i.e. unoxidized), pTE in the work-up distillation stage following oxidation and esterification (the so-called crude ester distillation) requires high energy consumption. The high content of the substances neutral to oxidation in the recirculated products is therefore accepted as a lesser evil, although they decrease the space-time yield and require an elevation in reaction temperature. Obviously, in plants having only low amounts of recirculated products, as described in DE-A-39 04 586, correspondingly higher amounts of pTE have to be conducted to the pX oxidation.
In the esterification stage, which follows oxidation, MMT and TPA, as well as pTA, are esterified with methanol, and the crude ester mixture, having DMT and pTE as major constituents, is separated by distillation. The crude DMT can be purified by further distillation and/or crystallization. From DMT, pure terephthalic acid (commercial designation PTA) which can be directly esterified with glycols, can also be produced by hydrolysis. It is characteristic of the Witten-Hercules process that pX and pTE in the mixture are oxidized and that pTA, TPA and MMT are esterified jointly.
Whereas the oxidation of pX to pTA is associated with only low yield losses, the oxidation of pTE to MMT or of pTA to TPA is the greatest source of loss in the process. This applies at least if, according to a conventional mode of operation, all of the pX and all of the pTE and, if desired, the above-noted recirculated products are fed to the start of the oxidation zone and are allowed to flow jointly together with the catalyst through the entire single- or multi-stage oxidation zone, in which they are oxidized to the desired degree of conversion, with or without multiple addition of fresh air.
The oxidation of pX to pTA proceeds at a considerably higher rate than the oxidation of pTE to MMT or of pTA to TPA. Therefore, at the beginning of the oxidation at moderate temperatures, such as 140.degree. to 145.degree. C., by far the greatest portion of the pX is first oxidized. There are narrow limits for the choice of oxidation temperature. Below 135.degree. C., the conversion rates decrease greatly, the reaction terminates readily and there are considerable safety risks because of possible oxygen break-throughs, with the explosion limit exceeded. Furthermore, at such low temperatures in the cooling systems of the oxidizers, only low-pressure steam can be generated, which is of restricted utility. Temperatures &gt;150.degree. C. are likewise disadvantageous, since at the customary reaction pressures of 6 to 8 bar, much pX and pTE are discharged with the off-gas and must be recovered. Additionally, at such elevated temperatures the oxygen is consumed so rapidly that the concentration falls below the desired oxygen concentration in the off-gas of at least 2%. Oxygen deficiency in the reaction mixture leads to increased formation of high-boilers, such as bi- and terphenyls, whereas high temperatures and excess oxygen promote total oxidation. Bi- and terphenyls constitute losses, since they cannot be converted into materials of value.
The oxidation of pTE and/or pTA to a significant extent does not succeed until the pX is substantially oxidized and, in addition, the temperature is increased by 10.degree. to 25.degree. C. The temperature must be further increased by approximately 10.degree. C. if, as done conventionally, the above-noted recirculated streams are introduced into the initial region of the oxidation zone together with pTE, or simultaneously with pTE. The losses due to total oxidation and formation of high-boilers then increase considerably. However, such temperatures are currently unavoidable in practice if satisfactory conversion rates are to be achieved.
In its simplest embodiment, the oxidation stage of the Witten-Hercules process operates batchwise. pX and pTE are introduced together with catalyst solution, the mixture is heated and air is introduced until the desired degree of oxidation is achieved. In the simplest continuous embodiment, pX, pTE, air and catalyst solution are fed to a single oxidizer and the oxidation mixture is continuously taken off. This embodiment operates at a defined preset temperature, and therefore does not take into account the requirements resulting from the differences in oxidation properties mentioned of the different constituents of the oxidation mixture.
A variant of the single-stage oxidation is described in JA-B2 62/14537. Its process provides two oxidation columns arranged in parallel, of which one is charged with a molar excess of pX and the other with a molar excess of pTE. The temperatures in the two parallel oxidizers can be the same or different and are preferably 150.degree. to 190.degree. C. This arrangement improves the yield of DMT versus the single oxidizer process. However, the publication does not give any teaching on the relationship between the composition of the oxidation mixture in the two oxidizers and the optimum temperature; and no temperatures are specified in the examples. Furthermore, the preferred temperature range is so high that total oxidation and formation of high-boilers occur to a considerable extent, as is also shown by the unsatisfactory yields in the tables accompanying the examples.
The most usual arrangement of oxidizers in continuous processes is that of two, or preferably three, oxidizers arranged in series (i.e., sequentially). In this arrangement the temperature in the individual oxidizers can be readily matched to the particular composition of the oxidation mixture and to other operating circumstances. It is widely common to conduct all of the pX and all of the pTE, with or without all of the recirculated products, through the entire oxidation stage which is divided by the oxidizers into part-stages.